Just a WAG (Wild A.. Guess) although I had a similar problem some years ago. You air inlet seems to almost flush right up against the top of your carb. That can cause airflow to smooth and flow right across the venturi inlet creating lesser air entering the carb which will cause a loading-up effect. With a smaller prop that may well provide the RPM and better suction for the carb to get better air flow into the carb.

Way back before dirt, with a CL Stunt model, I actually applied a short fence on front of the venturi to create an eddy which allowed the engine to perform much better, throughout the flight pattern. Only needed once in 20 years of competition CL stunt.
Also, if the engine backs up close to the firewall, even though there is a lot of open space below the engine, with just the right situation, the air develops a flow pattern that does not move around the cylinder head properly to allow for adequate cooling. Take-Off is a critical time, lots of power and little airflow. These are difficult items to pin down and can give one much frustration.

IMO, tune an engine in the environment that it will be operating in i.e., cowled.

I value all of you guys info... will try my best to take in consideration all opinions and options to try... firstly I have a 13x8 prop on there now... so I will also tune with the cowl on then as suggested and check the position and fuel lines of the tank vs the height of needle valve... this is what I am refering to when using the word valves or valve... hopefully this weekend I will have more luck...

You nevered answered when queried about having outlet air out the cowl bottom, and only spoke of having the cylinder head open. Unless there is substancial outlet in the bottom of the cowl back at the firewall then you will likely never have a successful flight if there is no outlet at the bottom you have crated a stagnant bake oven. Now couple this with unrealistic prop loads as the fellows are mentioning then you will never have any success.

John

there is air outlet... the cowl actually already catered for that... I didn't have to make extra openings... it was made for a pitts muffler... unrealistic prop load? meaning the 15x6?

Yes meaning the 15x6, You were typing at the same time last night and I see you have gone to a lighter load.

If its possible can you post a pic of the outlet on the cowl bottom, it may be inadaquate if you have not opened it enough. Something that is almost always neccessary and made worse if there is a pitts type muffler present. An in cowl muffer is just another heating element in the oven and something else to block airflow.

John

Having read the posts on this problem it seems that you have been given some good information. I can tell you of my experience with the OS 91 which I have flown many times. I used a 14X6 prop and had way to much prop noise. I then went to a 16X6 and flew the plane for many years without any problems. When I have had stalling trouble I have went threw all the sugestions that you have received and the final test is to go to full throttle and then hold the plane vertical. If too lean the engine will slow down if rich it will increase in speed. If the engine speed stays the same then open the high speed needle about two or three clicks as it will lean out some in the air. I was always happy with about 9000 to 10000 RPMS. When I did have a plane with a full cowl I had to remove about 1/4th of the cowl bottom to get enough air flow so it didn't heat up and have engine failure. I had the 91 on a Stick with a 6 foot wing and 4 foot fuse and no cowl.

hi have my 91fx on a 60 size airframe alsoso the engine runs mid range most of the flighti was on a 13/7 apcwent to a 13/6 apc to improve the mid range runit did (14/7apc did not work for me )-you must watch the fuel line before the carb on these enginesat midrange only you will see air in the line-it goes away at full boreif you have the engine with the back plate and needle valve as one casting -the design is the problemcontact OS they have a new back plate and nv for you free- peak the top end by ear and richen the nv almost 1/2 a turn on the top end (all my other engines require 1/4 turn rich here ) found that to help the mid range run a great deal i am sure you were overheating the engine on the 15/6 apc as i did also-when you reach top end RPM you must go richer to allow for the fuel drop in the tank during the flightwith this engine you must go richer than with most other engines

hi low end set up on the 91fx-run the engine about 10001500 RPMS back out the low end until it starts to run rough-turn it in until you get a real smooth run at that RPMthe low end is set to go-do this only after -you ran up the top end once-then do the top end again after the low end is done - do go richer than you would normaly on the top end adjustment on this engine you need to fly the plane to see the results i have been working with my 91fx to get it to run better for a month and a half i do have mine a long time10 years but about 50 flights -it has never gone dead stick but my problem is i just cannot live with the sound this engine makes in flightI JUST HATE ITbut the power is great

I had a similar issue with my Aviastar .53 and it turned out that it really likes (read ,will not run) on anythign but 0% nitro. Did you ask the previous owner what fuel he was using? I know that a friend at the field has a 1.08 OS that will only run on FAI fuel. My Webra .60 only likes FAI as well. If you have this handy, you might want to give it a try. It is a pain to carry multiple fuel types, but at least it is cheaper than 5 or 10. If you have any head shims for it, you might want to pop them in.
Good luck, I understand your frustration,
Curtis

well for this engine 10% (with castor) was recommended cause it protects better in lean runs... I am gonna richen it to around 8500 RPM on ground then and see if it finally wants to get into the air... but just a question... what is the difference of me holding it back at 45 degree angle in the air and revving it for 30s full throttle or so to make sure it doesn't cut out on me and the difference when I actually try and launch after I have reached full throttle and I have my speed to launch? cause I feel its the same... and also I hold it in a hover position and rev it to make sure also it doesn't die... these are all my checks before I try and launch... launch(takeoff)... but I will definitely richen it 800RPMS at least to see if it will finally launch...

I think the difference might be the load on the engine. When you are putting it up at a 45 degree angle, you are simulating the load on the fuel and making sure it is still getting to the engine, but you can't simulate the unloading of the prop. Richening the high speed needle should give you a little room for when the prop unloads, but doesn't simulate the effects on ignition timing. I would recommend the same mixture of oil and fuel, but if you can try some fuel without nitro, I would be curious to see how it runs. My Aviastar was also recommended to run with 10% nitro, but it didn't work so well.
Good Luck,
Curtis

I did not read your #11 post correctly about the plane having 15 previous flights...sorry about that.

The difference between holding the nose up while checking for proper needle adjustment AND the plane actually taking off and rotating it's nose up naturally is...a force created by the momentum of the forward traveling aircraft that wants to push the fuel to the back of the tank.

Check the fuel lines inside the tank, especially the main feed/pickup line near the brass tubing for a pinhole or slight nick or cut.

hi always ran my 91fx on 10 or 15 % morgan omega pinkwith castor oil- no problem i do use a k& B 1L GLO PLUG-I WILL be trying a four cycle plug in the future-do think about changing the "o " ring that is on your needle valve that was my problem at one time it looked ok but when replaced the engine did start to not have air in the fuel line just before the carb at mid rangedo the pinch test just before the carb on the low end and the top endthe RPMS should increase when the line is pinched in both places if you are rich in both places

Well, I thought that I would throw in my 5 cents worth here, I actually have the same engine in that position on a yak 54 of mine and also had a pitts muffler, and the one thing that I noticed is that the back pressure of the air rushing past the exhaust openings in flight becomes too great for the engine to compensate therefore it boggs and dies. This being my first larger engine I might be wrong, but when I changed back to the stock muffler the thing ran like a swiss watch.

I have read some guys saying to change the muffler... but I cant with this plane... else I need to change the mount position and tilt it about 45 then still I think the stock muffler will touch the firewall... the unloading of the prop and the fuel shifting back makes sense which was mentioned... I can only richen it up and hope for the best... once its in the air I don't need power at all... can cruise less than 50% throttle and still have enough speed... I opted with this engine cause its so light for a 90size... which is why I am using it in that 60 size plane...

jimmyjames,

I have a 91FX and reading from the manual..........for sport, OS recommends 15x8, 16x6 or 8, 17x6 or 14x7(3 blade).........for aerobatics they recommend 13x11 or 13 and 14x10 or 11. I'm flying mine with a 15x8 and it flys great.

Andy

+10 on the above!

Most likely situation given the descriptions...

my point exactly which is why I want to use a 15x6... for more torque and not so much speed... so it must work, it cant put too much stress on motor or be overloading... I must just go richer I guess and it should be awesome... what RPM are you getting with the 15x8?

I run 15X4 and 15X6 so I guess I better go smaller, well, maybe not, I like the way my plane fly's and my engine runs just fine with them?? I do open up most of the bottom of the cowl for air flow/cooling. Here is how I tune mine, I have a good tach so I used it, not really needed though. New OS#8 plug, fire up the engine, run it up to full throttle and lean out the top end to max then back it off about 400 RPM, cut the throttle to idle. Here is where it gets fun. I put the glow driver on the plug and let it idle for about 20 seconds then pull the driver, if the engine dsrops RPM then I put the driver back on the plug and lean in the low end then remove the driver again. I do this until there is no drop in RPM then I fatten up the low end until there is a slight RPM drop. That should do it?? If the engine boggs down on take off roll then don't take off, bring it back and fatten the high end a click or two. The Pitts muffler may not be giving you enough tank pressure so you can close up the holes in the pipes a little with JB weld but if the engine ran for someone else it seems like you are just out of tune.

here is my latest update... the engine seems to have nice response and it idles... I can open up full throttle from idle to wide open without issues... now comes the problem... I am at full throttle and I bring it down to round about mid... then the engine stutters... it hesitates to go back to full throttle, this will make the engine cut in air... so I didn't try fly it... in other words its good at idle and good at wide open... if I gradually increase throttle from that mid position it struggles... don't want my engine dying when I am doing a slow low pass and I want to open it up to pull me back up and it cuts... what could be the problem here?